1. Field of the Invention:
The present invention relates to a resilient support, or form block, made of low-hardness silicone elastomer, and well adopted for use in the stamping of sheet metal workpieces by the elastoforming of ultra-thin steel sheeting.
Although the description which follows more particularly focuses on the elastoforming of sheet metal, that is to say, of thin plates which are typically made of metal, it should nevertheless be understood that the elastoforming process employing the resilient supports according to the present invention is not limited in any way to this application. Thus, the subject elastoforming process may also be used for the forming of thin sheets of plastic materials, in particular for the forming of sheets of polybutene, polyethylene, chlorinated polyethylene, PVC, chlorinated PVC, ABS (acrylonitrile/butadiene/styrene), polycarbonates, polyphenylene oxide, polysulfones, polychlorotrifluoroethylene, cellulose acetate butyrate, cellulose acetate, nylons, nylon 6 and nylon 66. See, in particular, Polymer Engineering and Science, vol. 11, No. 2, page 106 (March, 1971).
Thus, in the present description and in the claims to follow, by the term "sheet metal" is intended a general meaning of "thin plate" or "sheet" without any intention to "metal products" only, but including all materials having a degree of plasticity, such as the metals and plastics.
More specifically, the resilient support made of silicone elastomer according to this invention is capable of being used in double-action (mechanical or hydraulic) presses such as, for example, those described in published European Patent Application EP-A-165,133, hereby incorporated by reference.
2. Description of the Prior Art:
The aforenoted '133 application describes a process for stamping sheet metal blanks of substantially constant thickness, hereinafter designated simply as metal sheets, in a double-action press, according to which process the metal sheets to be shaped are arranged on a resilient support, particularly of low-hardness silicone elastomer. A first external slide block, or blank-clamp, is applied to the peripheral part of the metal sheet, and then a second middle slide block is applied to the central part of the metal sheet; the peripheral part of this metal sheet is formed by being allowed to slide under the blank-clamp by means of at least one working portion of the external slide block such as to compensate in some regions of the finished article for the excess surfaces of unchanged thickness of the initial metal sheet in relation to the volume to be formed and, simultaneously, the middle slide block is moved to form the angular spaces of the middle part of the metal sheet by pressing the metal sheet against the surfaces of the middle part of the support. In a process of the above type, in the case where metal sheets having angular spaces and especially sheets of steel having a high elastic limit (for which E=kg/mm.sup.2) and which are ultra-thin (thickness below approximately 0.5 mm) are to be stamped in mass production, the low-hardness resilient support creeps in order to deform the middle area of the metal sheet, during the action of the working part of the external slide block on the metal sheet, such as to provide a surface which is substantially equal to the surface of the finished component to be produced. The middle slide block is then moved to form the angular spaces of the middle part of the metal sheet by final creep flow of the support.
In an alternate embodiment, a first external slide block is applied to the peripheral part of the metal sheet, and then a second middle slide block is applied to the middle part of the metal sheet; the peripheral part of the metal sheet to be formed is arranged on a lower blank-clamp forming a container for the resilient support and whose upper face for holding the metal sheet is situated at a level which is higher than that of the working face surface of the resilient support; the first external slide block, whose body has a diameter which is smaller than the lower blank-clamp and which at its periphery comprises an upper blank-clamp interacting with the lower blank-clamp in order to clamp the metal sheet, is applied, the descent of the external slide block is continued against the resilient support to produce the dropping of an edge of the metal sheet blank and creep flow of the mass of the resilient cushion is produced in order to deform the middle part of the metal sheet, such as to provide a surface which is substantially equal to the surface of the finished component to be produced, and then the middle slide block is moved to form the angular spaces and the middle portion of the metal sheet by final creep flow of the support. This alternate embodiment is the subject of French Patent Application No. 85/17,957, filed Dec. 4, 1985.
At page 11 of the aforesaid European Patent Application EP-A-No. 165,133 it is indicated that the resilient support should have a Shore hardness of less than 30 and preferably greater than 10, and that it may consist of a silicone elastomer, at least partly covered, if desired, with a relatively thin skin (for example, 10 to 15 mm) of a stronger and harder material, such as a silicone of Shore hardness 50, or of Teflon having advantageous anti-friction properties.
When the above-mentioned process is being used, the resilient support is subjected to stresses. The pressures may attain values of 500 bars and more. Pressures which are typically used range from 2 to 200 bars. The operating rates of the presses are typically from 10 to 60 strokes per minute. The support may furthermore be subjected to a volume deformation of 100%, which corresponds to a movement of 50% of the substance. In light of these conditions of use, this resilient support must have, in particular, the following properties:
(i) being free from the hazards and the limitations of the hydraulic fluids used for hydroforming under very high pressure;
(ii) being resilient with a high rate of relaxation after deformation (elastic recovery of less than one second);
(iii) having a low energy of deformation and a low hardness;
(iv) being very slightly compressible (degree of compressibility of less than 5% under 500 bars);
(v) being resistant to abrasion and to tearing;
(vi) having adequate fatigue and aging resistance;
(vii) having a sufficient heat resistance when heated by friction;
(viii) having a low heating effect during the deformations involved in the process;
(ix) not contaminating the formed surfaces, particularly when these surfaces are to be painted; and
(x) being conveniently handleable.
In said European Patent Application EP-A-No. 165,133 it is indicated that silicones of Shore hardness of less than 30 can be used, but no specific means are given permitting the production of a resilient silicone elastomer support of appropriate hardness and combining all of the properties delineated above.